You'd hardly guess it from all the silent acceptance of war as a legal enterprise and all the chatter about ways to supposedly keep war legal through the reform of particular atrocities, but there are international treaties that make wars and even the threat of war illegal, national constitutions that make wars and various activities that facilitate wars illegal, and laws that make killing illegal with no exceptions for the use of missiles or the scale of the slaughter.
Of course, what counts as legal is not just what's written down, but also what gets treated as legal, what is never prosecuted as a crime. But that's precisely the point of knowing and making more widely known the illegal status of war: to advance the cause of treating war as the crime that, according to written law, it is. Treating something as a crime means more than just prosecuting it. There may be better institutions in some cases than courts of law for achieving reconciliation or restitution, but such strategies are not assisted by maintaining the pretense of war's legality, war's acceptability.
Since 1899, all parties to the Convention for the Pacific Settlement of International Disputes have committed that they "agree to use their best efforts to insure the pacific settlement of international differences." Violation of this treaty was Charge I in the 1945 Nuremberg Indictment of Nazis. Parties to the convention include enough nations to effectively eliminate war if it were complied with.
Since 1907, all parties to the Hague Convention of 1907 have been obliged to "use their best efforts to ensure the pacific settlement of international differences," to appeal to other nations to mediate, to accept offers of mediation from other nations, to create if needed "an International Commission of Inquiry, to facilitate a solution of these disputes by elucidating the facts by means of an impartial and conscientious investigation" and to appeal if needed to the permanent court at the Hague for arbitration. Violation of this treaty was Charge II in the 1945 Nuremberg Indictment of Nazis. Parties to the convention include enough nations to effectively eliminate war if it were complied with.
Since 1928, all parties to the Kellogg-Briand Pact (KBP) have been legally required to "condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another," and to "agree that the settlement or solution of all disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them, shall never be sought except by pacific means." Violation of this treaty was Charge XIII in the 1945 Nuremberg Indictment of Nazis. The same charge was not made against the victors. The indictment invented this previously unwritten crime: "CRIMES AGAINST PEACE: namely, planning, preparation, initiation or waging of a war of aggression, or a war in violation of international treaties, agreements or assurances, or participation in a common plan or conspiracy for the accomplishment of any of the foregoing." This invention strengthened the common misunderstanding of the Kellogg-Briand Pact as a ban on aggressive but not defensive war. However, the Kellogg-Briand pact clearly banned not only aggressive war but also defensive war - in other words, all war. Parties to the Pact include enough nations to effectively eliminate war by complying with it.
Since 1945, all parties to the UN Charter have been compelled to "settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered," and to "refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state," albeit with loopholes added for UN-authorized wars and wars of "self-defense," (but never for the threatening of war) loopholes that do not apply to any recent wars, but loopholes the existence of which create in many minds the vague idea that wars are legal. The requirement of peace and ban on war has been elaborated over the years in various UN resolutions, such as 2625 and 3314. The parties to the Charter would end war by complying with it.
Since 1949, all parties to NATO, have agreed to a restatement of the ban on threatening or using force found in the UN Charter, even while agreeing to prepare for wars and to join in the defensive wars waged by other members of NATO. The vast majority of the Earth's weapons dealing and military spending, and a huge portion of its war making, is done by NATO members.
Since 1949, parties to the Fourth Geneva Convention have been forbidden to engage in any violence toward individuals not actively engaged in war, and banned from all use of "[c]ollective penalties and likewise all measures of intimidation or of terrorism," while meanwhile the vast majority of those killed in wars have been non-combatants. All the big war makers are party to the Geneva Conventions.
Since 1970, the Treaty on the Non-Proliferation of Nuclear Weapons has required its parties to "pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a treaty on general and complete disarmament under strict and effective international control." Parties to the treaty include the biggest 5 (but not the next 4) possessors of nuclear weapons.
Since 1976, the International Covenant on Civil and Political Rights (ICCPR) and the International Covenant on Economic, Social, and Cultural Rights have bound their parties to these opening words of Article I of both treaties: "All peoples have the right of self-determination." The word "all" would seem to include not only Kosovo and the former parts of Yugoslavia, South Sudan, the Balkans, Czechia and Slovakia, but also Crimea, Okinawa, Scotland, Diego Garcia, Nagorno Karabagh, Western Sahara, Palestine, Southern Ossetia, Abkhazia, Kurdistan, etc. Parties to the Covenants include most of the world.
The same ICCPR requires that "Any propaganda for war shall be prohibited by law." (Yet the prisons are not emptied out to make room for the media executives. In fact, whistleblowers are imprisoned for revealing war lies.)
Since 1976 (or the time of joining for each party) the Treaty of Amity and Cooperation in Southeast Asia (to which China and various nations outside of Southeast Asia, such as the United States, Russia, and Iran, are party) has required that:
"In their relations with one another, the High Contracting Parties shall be guided by the following fundamental principles:
a. Mutual respect for the independence, sovereignty, equality, territorial integrity and national identity of all nations;
b. The right of every State to lead its national existence free from external interference, subversion or coercion;
c. Non-interference in the internal affairs of one another;
d. Settlement of differences or disputes by peaceful means;
e. Renunciation of the threat or use of force;
f. Effective cooperation among themselves. . . .
"Each High Contracting Party shall not in any manner or form participate in any activity which shall constitute a threat to the political and economic stability, sovereignty, or territorial integrity of another High Contracting Party. . . .
"The High Contracting Parties shall have the determination and good faith to prevent disputes from arising. In case disputes on matters directly affecting them should arise, especially disputes likely to disturb regional peace and-harmony, they shall refrain from the threat or use of force and shall at all times settle such disputes among themselves through friendly negotiations. . . .
"To settle disputes through regional processes, the High Contracting Parties shall constitute, as a continuing body, a High Council comprising a Representative at ministerial level from each of the High Contracting Parties to take cognizance of the existence of disputes or situations likely to disturb regional peace and harmony. . . .
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